SOx produced from the burning of organic sulfur-containing compounds present in fuel oils not only can cause acid rain, pollute air, and harm human health, but also can poison irreversibly the three-way catalysts in the tail gas cleanup systems of engines. Therefore, it has been receiving much attention. The refining of distillates is an essential and important process for removing the heteroatoms such as sulfur, nitrogen and the like present in fuel oils in order to meet the requirements on environmental protection. A conventional refining process is hydrorefining, that is to say, the transformation of the organic sulfur-containing and the organic nitrogen-containing compounds by hydrogenation into H2S and NH3, respectively to achieve the goal of removing sulfur and nitrogen. Although the conventional hydrorefining process is effective, the following problems still exist: 1. High investment costs because of setting up the high temperature and high pressure treatment units and the units for hydrogen production (if there is no reforming hydrogen) and sulfur recovery, 2. High operating costs due to high temperature, high pressure, and hydrogen consumption during the process, 3. Environmental pollution due to toxic H2S produced in the process, and 4. Hydrorefining being very effective in removing the sulfur-containing compounds of non-thiophenes, but being very difficult in removing the sulfur-containing compounds of thiophenes, especially, benzothiophene and dibenzothiophene and their derivatives. It is absolutely necessary to reclaim the existing hydrorefining units and to increase operating temperature and hydrogen partial pressure to remove the sulfur-containing compounds. However, it is certain that this brings about a number of problems including high investment, high operating cost, reduction of the catalyst cycle length, and reduction of the quality of the oil products. As the environmental protection regulations are increasingly strict, there is an urgent need for a more economic, more environmental friendly, and more effective production method of ultra-low sulfur diesel (having a sulfur content of less than 10 ppm sulfur).
U.S. Pat. No. 6,160,193 discloses a desulfurization method for distillate oil, which comprises selectively oxidizing the sulfur-containing and nitrogen-containing compounds present in the distillate oil into the corresponding oxides of sulfur and nitrogen compounds to increase the polarity thereof, and then extracting the oxides by using an solvent which is immiscible with the hydrocarbons in the distillate oil to achieve the goal of desulfurization and denitrogenation. The strong oxidants used in the patent are strong acidic oxidants such as perboric acid, persulfuric acid and the like and the reaction temperature is relatively high. The process inevitably causes the side reaction and damages the quality of diesel. In addition, the strong acidic oxidants are strong corrosive to the equipment and are environmental unfriendly as well.
U.S. Pat. No. 6,274,785 improves the above patent through using peracetic acid as oxidant instead of the strong acidic oxidant. However, it is unfavorable for decrease of the operating cost due to using a relatively large amount of peracetic acid as oxidant consumed during the process. In the meantime, peracetic acid is very strong corrosive to the equipment and is environmental unfriendly as well.
U.S. Pat. No. 6,402,940 discloses a desulfurization method for distillate oil, which comprises reacting a sulfur-containing fuel oil with a mixture solution of oxidant and extractant (a mixture solution consisting of a small amount of hydrogen peroxide, a large amount of formic acid and less than 25 wt % of water) at 50-130° C., and then separating aqueous solution from oil, and then removing a small amount of the residual formic acid and water by distillation, and drying the oil by using calcium oxide, and then removing the sulfone left in the oil by passing the obtained oil through a adsorption column with alumna adsorbent. This desulfurization process is environmental unfriendly because of using a large amount of formic acid. In addition, the operating temperature is relatively high, and there are a lot of side reactions. This not only consumes a great deal of hydrogen peroxide, and increases operating cost, but also damages the quality of petroleum products (for example, colority etc.).
An aqueous-organic biphasic system is used in the above three patents so that the reaction rate is slow. The reaction rate in the biphasic systems is often increased by increasing the reaction temperature.
Other methods are also used to improve the reaction rates. For example, U.S. Pat. No. 6,402,939 and U.S. Pat. No. 6,500,219 disclose a desulfurization method for a fuel oil, which comprises combining the fuel oil with an alkyl hydroperoxide (or hydrogen peroxide), a surfactant, and a metal catalyst (for example molybdate etc.) to form an mixture, and placing a container containing the mixed solution into an ultra-sound generator at a temperature of 75° C., oxidizing the sulfur-containing compounds in the oil into sulfone or/and sulfoxide, extracting the sulfone or/and sulfoxide from the oil by using a polar extractant to achieve the goal of desulfurization. Although the ultra-sound, the surfactant, and the catalyst used in the patent can accelerate the reaction rates; the reaction temperature is still relatively high (>70° C.). This inevitably brings about a lot of side reactions, consumes a great deal of hydrogen peroxide, increases the operating cost, and damages the quality of diesel (for example, colority etc.) as well. Moreover, the patent does not mention how to recover the surfactant and the catalyst.
U.S. Pat. No. 5,958,224 also discloses a process for deep desulfurization by combining a hydrotreatment with an oxidation, which comprises reacting the sulfur-containing compounds in the hydrotreated fuel oils with a peroxy-metal complex oxidant, oxidizing the sulfur-containing compounds into sulfone, and then removing the sulfone by adsorbing through an adsorption column with an adsorbent. Because the catalyst used in the process is immiscible with the fuel oil, it can be only dissolved in chloro-containing hydrocarbons (such as CH2Cl2), which is environmental unfriendly due to its toxicity.
Chinese Patents (having application Nos. of 03107599.1, 200510073771.4, 200610001691.2, 200610001376.X, 200610011979.8) disclose a process for ultra-deep desulfurization of a pre-hydrorefined diesel oil, which comprises agitating well the mixture consisting of the hydrorefined diesel, an aqueous hydrogen peroxide solution, and an amphiphilic catalyst, transforming the sulfur-containing compounds in the diesel into sulfone or/and sulfoxide under relatively mild reaction conditions (at a reaction temperature ≦60° C., at atmospheric pressure) to obtain an oxidized diesel, removing selectively the most of sulfone or/and sulfoxide form the oxidized diesel through a polar extractant to obtain ultra-low sulfur diesel. At present, some challenging problems still exist in oxidation-extraction desulfurization processes: 1. High installation investment costs and raw materials costs necessary for separation and recovery of extractants; 2. Extractants involved including nitrogen-containing compounds, such as acetonitrile, dimethylformamide etc., which is environmental unfriendly; 3. Low yield of ultra-low sulfur diesel; and 4. Difficult to recover catalyst.